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How to Address Defects in UV 3C Coatings (Part 10)

Release time:

2026-07-03 16:04

How to Address Defects in UV 3C Coatings (Part 10)

Among the various defects in UV‑3C coatings, poor adhesion is a serious issue that compromises coating reliability. It manifests as easy delamination of the paint film from the substrate—often removable with a simple tape test—indicating insufficient interfacial bonding strength between the coating and the substrate. Adhesion is the foundation for the coating’s protective performance; inadequate adhesion directly results in the failure of the coating’s protective function. To address this defect, appropriate measures must be implemented across multiple aspects, including substrate cleaning, surface activation treatments, control of curing conditions, and compatibility of coating systems. This paper outlines strategies for mitigating poor adhesion by focusing on pre‑treatment process optimization, substrate surface modification, adjustment of curing parameters, and alignment of coating system components.

I. Cleaning and Preparation of the Substrate Surface

Residual release agents and oil contamination on the substrate surface are the primary causes of poor adhesion. Prior to coating, the workpiece must be thoroughly cleaned. Release agent residues can be particularly stubborn and require treatment with a dedicated release‑agent remover. The cleaning method should be selected based on the part’s geometry and the extent of contamination, and may include solvent wiping, ultrasonic cleaning, or spray washing.

The removal of oil and grease is equally important. Alkaline cleaners or solvent-based cleaners can be used to remove surface oils from the substrate; after cleaning, rinse with deionized water to eliminate any residual cleaning agent. The cleaned parts should be thoroughly dried to prevent moisture residues from compromising adhesion.

After cleaning, workpieces should be protected from secondary contamination during storage and handling. Operators must wear clean gloves to prevent direct contact between bare hands and the workpiece surface. Cleaned workpieces should be coated within the specified time frame to avoid prolonged exposure to the environment, which could lead to the adsorption of contaminants.

II. Surface Activation Treatment of Low-Surface-Energy Substrates

PP, PE, and other low‑surface‑energy substrates have inherently low surface energies, making it difficult for primers to wet and adhere directly. To address this, the substrate must undergo surface activation to increase its surface energy. Plasma treatment is a widely used approach: by bombarding the substrate surface with plasma, surface contaminants are removed and polar functional groups are introduced, thereby enhancing surface energy and wettability.

Corona treatment is suitable for flat substrates, using high‑voltage discharge to introduce polar functional groups onto the substrate surface, thereby increasing surface energy. For parts with complex geometries, a compatible primer can be applied. The primer forms a high‑surface‑energy film on the substrate, enhancing wetting between the coating and the substrate.

The effectiveness of surface treatment can be assessed by contact angle measurements, ensuring that the treated surface meets the wetting requirements of the primer. The treated substrate should be coated as soon as possible to prevent a decline in surface energy over time.

III. Control of the Curing State

The curing state directly affects adhesion. During processing, curing parameters must be optimized to prevent under‑curing or over‑curing. Incomplete curing results in insufficient crosslink density and low cohesive strength, leading to poor adhesion. It is essential to ensure that the curing energy and irradiation time meet the requirements of the coating formulation, and to regularly verify the output energy of the lamp.

When curing is excessive, internal stresses in the coating increase, leading to heightened brittleness and, consequently, reduced interfacial adhesion. The curing energy should be optimized according to the coating’s properties to prevent over‑crosslinking caused by excessively high energy levels. The degree of cure can be verified through adhesion and hardness tests, enabling the identification of optimal curing conditions that balance insufficient and excessive curing.

IV. Optimization of Coating–Substrate Compatibility

The compatibility between the coating and the substrate affects adhesion. During application, it is essential to select a matching coating system based on the substrate type. Different substrates exhibit variations in chemical composition and surface characteristics; a coating formulation suitable for ABS may not be appropriate for PC or PP substrates. Choose a primer that is compatible with the substrate to ensure proper compatibility between the primer and the substrate.

The hardness and film thickness of the primer must also be carefully controlled. When the primer’s hardness is too high, the large difference in elastic modulus between the coating and the substrate leads to stress concentration at the interface. The primer film thickness should be maintained within an appropriate range: if it is too thick, internal stresses increase; if it is too thin, adhesion is inadequate.

Silicone‑based additives should be avoided in primer formulations, or their dosage should be carefully controlled, to prevent excessively low surface tension from compromising the adhesion of subsequent coating layers.

V. Localization of Adhesion Failure and Targeted Remediation

The location of adhesion failure helps identify the root cause and enables targeted corrective measures. When failure occurs at the coating–substrate interface, the fracture surface is smooth, indicating insufficient bonding between the primer and the substrate; in such cases, particular attention should be paid to verifying that the substrate has been properly cleaned and adequately surface‑activated.

When failure occurs within the coating, residual coating material can be observed on the delamination surface, indicating insufficient cohesive strength of the coating. This may be related to incomplete curing, and the curing parameters should be carefully verified to ensure they meet the specified requirements.

When failure occurs at the interface between the primer and the topcoat, it indicates insufficient intercoat adhesion, which may be attributable to surface contamination of the primer, excessive curing of the primer, or paint incompatibility. The primer’s curing condition and intercoat compatibility should be inspected.

VI. Integrated Process Control

Addressing poor adhesion requires a comprehensive approach that integrates multiple stages, including pretreatment, surface activation, cure control, and coating formulation. For pretreatment, thoroughly remove release agents and oil contaminants; for surface activation, apply plasma or corona treatment to low‑surface‑energy substrates; for curing, carefully regulate the degree of cure to prevent under‑curing or over‑curing; and for coatings, select a compatible system that matches the substrate.

The controls at each stage are interrelated and must be considered holistically during adjustments. In actual production, the primary source of adhesion failure can be identified based on its characteristic manifestations, allowing for targeted reinforcement of the corresponding process steps.

VII. Conclusion

Addressing poor adhesion involves multiple steps, including substrate cleaning, surface activation, control of the curing state, and optimization of coating compatibility. By thoroughly removing release agents and oil contaminants, subjecting low‑surface‑energy substrates to plasma or corona treatment, carefully managing the degree of cure to avoid under‑ or over‑curing, and selecting a coating system that is well matched to the substrate, adhesion can be significantly improved. Analyzing the location of adhesion failures helps identify the underlying cause, enabling the implementation of targeted corrective measures. Optimizing each of these stages requires coordinated efforts, with careful consideration of substrate condition, equipment capabilities, and material properties, in order to achieve an ideally satisfactory level of adhesion.

Disclaimer: The above content has been compiled from publicly available sources and is provided for reference only. If any infringement occurs, please contact us, and we will address it promptly.

Bosheng Related Product Recommendations – 3C Coatings
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B-619W	Aliphatic polyurethane acrylate	Fast curing, high hardness, excellent toughness, wear resistance, and chemical resistance.
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BM4241 (DiTMPTA-80)	Bis(2,3-dihydroxypropyl) tetraacrylate	High crosslink density, high hardness, chemical and wear resistance, and water resistance.
BM4242 (Di-TMPTA)	Bis-trimethylolpropane tetraacrylate	High crosslink density, high hardness, chemical and wear resistance, and water resistance.
BM6261 (DPHA-80)	Dipentaerythritol hexaacrylate	High crosslink density, high hardness, chemical and wear resistance, and water resistance.
BM6263 (DPHA-90)	Dipentaerythritol hexaacrylate	High crosslink density, high hardness, chemical and wear resistance, and water resistance.

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How to Address Defects in UV 3C Coatings (Part 11)

How to Address Defects in UV 3C Coatings (Part 9)